Summary The response of cortical cells to sensory stimulation reflects the interactions between feedforward thalamocortical input and ongoing cortical activity which can be strongly affected by behavioral states. Until recently, it has been experimentally challenging to tease apart the thalamocortical and intra-cortical pathways and their unique contributions to the sensory response. We propose to use intracellular recording together with optogenetic silencing of cortical activity to examine how thalamocortical (TC) and intracortical (IC) inputs combine to produce three hallmarks of the cortical sensory response: 1) trial-to-trial variability in the sensory response, 2) noise correlations between cortical cells, and 3) state-dependent modulation of response variability and gain. To address these questions, we will record intracellularly from layer 4 (L4) neurons of visual cortex and compare the sensory-evoked synaptic input to these cells with and without optogenetic silencing of cortical activity. It has been suggested that modulation by attention and arousal are mediated by suppression of noise correlation. The proposed experiments will determine how much variability of cortical neurons and their cross correlation are inherited from thalamocortical inputs, and whether that variability and correlated spiking are amplified or diminished by cortical processing and by behavioral state.